axis of symmetry

A line drawn through a geometric
figure such that the figure is symmetrical (see symmetry)
about it. The line may be considered as an axis of rotation: if the figure
is rotated about it, there will be two or more positions which are indistinguishable
from each other. If the letter Z, for example, is rotated about an axis
drawn perpendicularly into the paper through the center of the diagonal
stroke, there will be two correspondent positions: the letter has 2-fold
rotational symmetry about that axis. In general, if a figure has n
correspondent positions on rotation about an axis, it is said to have n-fold
symmetry about that axis.